1. Field of the Technology
The present disclosure generally relates to hot isostatic pressing. Certain aspects of the present disclosure relate to canisters and methods for hot isostatic pressing.
2. Description of the Background of the Technology
Hot isostatic pressing, which is often referred to by the shorthand “HIPping”, is a manufacturing process for making large powder metallurgy articles, including, but not limited to, large cylinders. HIPping conventionally is used to consolidate metal and metal alloy powders into powder canister forging compacts, which may be cylindrical or have other billet shapes. The HIPping process improves the material's mechanical properties and workability for subsequent forging and other processing.
A typical HIP process includes loading powdered metal and/or powdered metal alloy (“metallurgical powder”) into a flexible membrane or a hermitic canister, which acts as a pressure barrier between the powder and the surrounding pressurizing medium. The pressurizing medium may be a liquid or, more commonly, an inert gas such as argon. In HIP processes in which a canister is used, the powder-loaded canister is placed in a pressure chamber and heated to a temperature at which the metallurgical powder inside the canister forms metallurgical bonds. The chamber is pressurized and held at high pressure and temperature. The canister deforms, and the metallurgical powder within the canister is compressed. The use of isostatic pressure ensures a uniform compaction pressure throughout the mass of metallurgical powder, which results in a homogeneous density distribution in the consolidated compact.
A HIPping canister may have a cylindrical shape or any other desired shape suitable for forming the desired compacted shape from metallurgical powder placed in the canister. One conventional HIPping canister design, shown schematically in FIG. 1A as canister 100, includes a cylindrical steel wall and flat or stepped endplates. FIG. 1B is a schematic representation of a cross-section through the central axis of a portion of HIPping canister 100. HIPping canister 100 includes a body portion 102 and flat endplates 104 secured to each end of the body portion 102 by weld beads 106. Fill stems 108 are secured through the endplates 104 and are configured to allow the canister 100 to be filled with the metallurgical powder and allow for air to be evacuated from the canister 100. Once canister 100 is filled with the metallurgical powder and air is evacuated from the canister 100, the canister 100 is sealed. Sealing may be accomplished by crimping the fill stems 108 or by other means isolating the interior of the canister 100 from the external environment. The body portion 102, endplates 104, and fill stems 108 are typically made from mild steel or stainless steel.
Conventional HIPping canister designs have several disadvantages. For example, it is difficult to clean the interior of conventional cylindrical HIPping canisters after assembly. Also, it may not be possible to completely fill the interior of a conventional HIPping canister with metallurgical powder due to the difficulty in moving the powder horizontally after it enters the canister through a fill stem. Certain HIPping canisters designs include multiple fill stems to improve canister filling and enhance degassing efficiency. Including additional fill stems, however, adds cost, provides additional points of possible canister failure during HIP, and typically has only a small effect on increasing vacuum degassing efficiency. Welds securing fill stems through the endplates (and securing the endplates to the canister body) are under extreme stress during HIP consolidation due to locally high distortion, and including multiple fill stems to address powder fill problems increase the risk of weld failure during HIP consolidation. Also, conventional canister designs including multiple fill stems must be inverted during HIPping to ensure that all stems are filled with metallurgical powder and to prevent stem collapse during consolidation, and this procedure increases risk to personnel and creates an opportunity for part damage.
Accordingly, there is a need for an improved HIPping canister design. Such a design preferably addresses powder filling problems associated with conventional canister designs, but without a requirement for including additional fill stems on the canister.